1. Field of the Invention
The present invention relates to tilt detection system, tilt detection method and tilt adjustment method in an optical disk drive for detecting/adjusting a tilt of a light beam with respect to a disk recording surface.
2. Description of the Related Art
A light beam emitted from a semiconductor laser is incident on a recording surface of an optical disk at right angles through an objective lens for reading a signal recorded on the optical disk. However, when the light beam is not incident on the recording surface of the optical disk at right angles due to inclination of the optical disk and/or inclination of the objective lens, coma aberration is generated. As the coma aberration increases, the cross talk increases, and the S/N ratio of the read signal is degraded. Thereby, distortion occurs in the reproduced waveform, and jitter occurs, as is well known.
Therefore, in order to adjust the tilt of light beam with respect to the optical disk, ordinarily, the jitter amount of the RF waves reproduced from the optical disk is measured for radial direction and tangential direction of the optical disk (hereinafter, the term ‘of the optical disk’ in ‘radial/tangential direction of the optical disk’ being omitted) separately, with the optical disk in the reproduced condition, and the respective tilts are adjusted so that the jitter amount becomes minimum. This adjustment may be made as a result of the pickup of the optical disk drive being inclined or as a result of the spindle motor of the optical disk drive being inclined.
FIGS. 1A and 1B show the jitter amount of the reproduced RF signal with respect to the tilt in radial direction and the jitter amount of the RF signal with respect to the tilt in tangential direction, respectively, at the time of tilt adjustment. The jitter amount is minimum when the tilt is 0, and the jitter amount increases as the tilt increases. The inclination of the pickup or spindle motor for radial direction and tangential direction is adjusted, where the state (inclination) of the pickup or spindle motor in which the jitter amount is minimum is regarded as the optimum state in tilt adjustment.
As shown in FIG. 1B, change in jitter amount with respect to the tilt in tangential direction is steep about the point at which the tilt is 0. Accordingly, it is easy to find the optimum state in tilt adjustment, and, thus, to adjust the tilt optimally. However, as shown in FIG. 1A, change in jitter amount with respect to the tilt in radial direction is gentle about the point at which the tilt is 0. Accordingly, it is difficult to find the optimum state in tilt adjustment, and, thus, to adjust the tilt optimally.